Resized images look flat in entries

I use twenty-twelve theme
After uploading images (1280pix wide) into Image Library and adding them in a new entry they are displayed at 625 pix wide. Compared to the original photo they look poor, flat, degraded color.
(Looking at the html output I see a new file (1024pix wide) is created after uploading the original photo and displayed at 625pix wide “width=625 he…”).
How can I solve this?
Here’s a SAMPLE